Ever more stringent legal regulations with regard to admissible pollutant emissions of motor vehicles comprising internal combustion engines make it necessary to keep the pollutant emissions during the operation of the internal combustion engine as low as possible. This may firstly be achieved by reducing pollutant emissions generated during the combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. Secondly, in internal combustion engines, use is made of exhaust-gas aftertreatment systems which convert the pollutant emissions generated during the combustion process of the air/fuel mixture in the respective cylinders into non-harmful substances. In this connection, use is also made for example of diesel particle filters.
To control the combustion process in the cylinder, or also the conversion process in the exhaust-gas aftertreatment system, in a targeted manner, in each working cycle of a cylinder, the fuel is metered in in an injection cycle in relation to the rotational angle of the crankshaft, composed of a plurality of temporally spaced-apart partial injections which differ in quantity. Depending on the number of cylinders and the extent of the individual injection cycles over the rotational angle of the crankshaft, temporal overlapping of injection cycles of individual cylinders which follow one another in the ignition sequence may arise. This means that, in the worst case, individual partial injections for different cylinders can temporally overlap. However, if only one common output stage is available for actuating the injectors of the cylinders in question, this leads to a conflict because in each case only one injector of the output stage group defined in this way can be actuated. This problem arises in particular when, in specific operating modes, certain partial injections must be carried out very late in the working cycle of a cylinder in order to supply unburned air/fuel mixture to the exhaust-gas aftertreatment system. This is necessary for example in order, on demand or at certain time intervals during ongoing operation, to increase the temperature in the exhaust-gas aftertreatment system for the regeneration of the filters or catalytic converters.
Said late partial injections, which fall temporally, or in relation to the crankshaft angle, within the injection cycle of the subsequent cylinder of the same output stage group in the ignition sequence, will be referred to hereinafter for short as late injections.
The partial injections arranged in the crankshaft angle range or time period between the start of the respective injection cycle and before the start of the subsequent injection cycle, that is to say before the start of the earliest partial injection of the injection cycle of the subsequent cylinder of the same output stage group in the ignition sequence, will be referred to hereinafter as working injections.
DE 10033343 A1 discloses a fuel injection system for an internal combustion engine having at least two cylinders. The fuel injection system has at least two piezoelectric actuator elements, and each cylinder is assigned at least in each case one piezoelectric actuator element for the injection of fuel into the cylinders by the charging or discharging of the piezoelectric element. The fuel injection system has an injection regulating means for monitoring and/or resolving a conflict in the actuation of actuator elements for injecting fuel into the respective cylinders.
In a method known from EP 1 497 544 B1 for operating a fuel injection system, it is monitored whether, during an injection of relatively low priority, the charging or discharging of the associated piezoelectric element takes place within a predefined time interval around the time of a charging or discharging of a further piezoelectric element for an injection of relatively high priority. During the operation of the fuel injection system, the intervals between temporal charging and/or discharging flanks are determined, and from this the magnitude of a displacement and/or shortening of the injections of relatively low priority in relation to the injections of relatively high priority are determined.